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    Àý£º  create table student(
                 xh number(4),
                 xm varchar2(20),
                 sex char(20),
                 birthday date,
                 sal number(7,2)
            );
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    alter table student add(classId number(2));
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     desc student;
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    alter table student modify (xm varchar2(30))
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    alter table student modify(xm char(30));
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alter table student drop column sal£»
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 rename student to stu;
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drop table student;